Vitamin poisoning, hypervitaminosis or vitamin overdose refers to a condition of high storage levels of vitamins, which can lead to toxicsymptoms. The medical names of the different conditions are derived from the vitamin involved: an excess of vitamin A, for example, is called "hypervitaminosis A."

With few exceptions, like some vitamins from B complex, hypervitaminosis usually occurs more with fat-soluble vitamins, which remain more time in the body and are harder to be excreted than water soluble vitamins.

High dosage vitamin A; high dosage, slow release vitamin B3; and very high dosage vitamin B6 alone (i.e. without vitamin B complex) are sometimes associated with vitamin side effects that usually rapidly cease with supplement reduction or cessation.

Vitamin C has a brief, pronounced laxative effect when taken in large amounts, typically in the range of 5-20 grams per day in divided doses for a person in normal "good health," although seriously ill people,[1] may take 100-200 grams without inducing vitamin poisoning.

High doses of mineral supplements can also lead to side effects and toxicity. Mineral-supplement poisoning does occur occasionally due to excessive and unusual intake of iron-containing supplements, including some multivitamins, but is not common.

Death by vitamin poisoning appears to be quite uncommon in the US, typically none in a given year. Before 1998, several deaths per year were associated with pharmaceutical iron-containing supplements, especially brightly-colored, sugar-coated, high-potency iron supplements, and most deaths were children.[2] Unit packaging restrictions on supplements with more than 30 mg of iron have since reduced deaths to 0 or 1 per year.[2] These statistics compare with 59 deaths due to aspirin poisoning in 2003 [3] and 147 deaths associated with acetaminophen-containing products in 2003.[3]